1. Field of the Invention
The present invention relates to a mobile communication system, an extension transmission/reception server, an extension transmission/reception controller, a base station, a radio network controller, a mobile station and a communication method.
2. Description of the Related Art
A conventional mobile communication system 400 consists of a plurality of MSs (mobile station) 410a and 410b, which connects to a plurality of BSs (base station) 420a and 420b, a radio access network 450 and a core network 480 conforming to the 3GPP (Third Generation Partnership Project), as shown in FIG. 1. The radio access network 450 consists of a plurality of RNCs (radio network controller) 440a and 440b and the plurality of BSs 420a and 420b, which connects to the RNCs 440a and 440b. The core network 480 consists of an MSC (mobile switching center) 460, which connects to the RNCs 440a and 440b, and a HLR (home location register) 470, which connects to the MSC 460 (“Textbook for wireless broadband”, under editorship of Takeshi Hattori and Masanobu Fujioka, IDG Japan Inc., Jun. 10, 2002; pp. 26-37).
In the mobile communication system 400, the communication is carried out in the procedure shown in FIG. 2. FIG. 2 shows a case where the MS 410a, which exists in the radio zone of the BS 420a, is the communication source; and the MS 410b is the communication destination. When starting communication, the MS 410a connects to the RNC 440a via the BS 420a. And the MS 410a transmits a call request to the RNC 440a (S401). The RNC 440a inquires the MSC 460 whether or not the connection to the communication destination MS 410b is available (S402). The MSC 460 retrieves the HLR 470, and obtains information about the MS 410b from the HLR 470 (S403).
Based on the obtained information about the MS 410b, the MSC 460 determines whether or not the RNC 440a can connect to the communication destination MS 410b via the BS 420b, the RNC 440b and the MSC 460 to which the MS 410b connects. When it is determined as connectable, the MSC 460 instructs the RNC 440a to connect to the MS 410b (S404). Also, the MSC 460 calls the MS 410b and receives a call response from the MS 410b (S405).
When receiving the instruction, the RNC 440a connects to the BS 420a (S406). The MS 410a transmits user data for the MS 410b to the BS 420a; thus the communication is started (S407). Accordingly, as shown in FIG. 3, in the mobile communication system 400, a path C of the user data always goes through the MSC 460. That is, in the mobile communication system 400, every call is processed via the MSC 460.
Therefore, even when the communication destination MS and the communication source MS exist under the control of the same BS, the user data has to go through such redundant path that goes via the MSC. Therefore, many lines for that are required; and line use fee for the paths that go via the MSC; and delay due to those paths is resulted in. Ordinarily, in many cases, in order to integrate the functions, the RNC and the MSC are located in a place away from the BS. In such case, the above problem appears remarkably.
To cope with the above problem, the following technique has been proposed. That is, MSCs, which have, in the functions of the conventional MSC, a function to process the communication with the MSs existing under the control of one BS or a small number of BSs, are distributed in places near these BSs (hereinafter, referred to as “distribution MSCs”). In this case, in a hierarchy higher than the distribution MSCs, a MSC, which has the function of the conventional MSC, is provided (hereinafter, referred to as “integration MSC”). Owing to this, when both of the communication source MS and communication destination MS exist under the control of the same distribution MSC, the path for the user data can be reduced. Accordingly, in this case, the line use fee for the path via the MSC, which is counted when only the conventional MSC is available, can be reduced, and the delay due to the path can be reduced.